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On the inline thiers about 5 conecters bolted to the valve over under the egr it may be one of them or it could be a plug thatis used on a differnt model that yours does not use that just finally came into view
Look at the back of the valve cover on the drivers side, near your fuel lines. There is a plate that is bolted to the valve cover and has several connectors on it. On my '90, there are three...a large gray one, a small black one, and a small gray one. The small gray one is at the very back, and that is where I think that connector goes on your truck.
It could very well be that it isn't used on your truck, but it looks way too clean to have been just hanging since it was made.
Double check those plugs on the valve cover and let us know for sure. The only other plug I see on mine which is that size is on the IAC, at the top of the throttle body...but it is black.
I will get back into it tomorrow after work but before the super bowl. I didn't look at it today, the Wife and kids needed some attention. Tomorrow is a new day and everyone is happy right now. I should be able to spend the afternoon outside and the evening drinking and watching the game.